New Orleans officials ordered a mandatory evacuation Sunday as Hurricane Katrina bears down on the vulnerable city.

Katrina was Category Five hurricane packing winds of nearly 175 miles per hour Sunday morning, but what worries New Orleans officials more is the storm surge.

The city is below sea level in what amounts to a bowl that could be a death trap for any residents who chose to try to ride out the storm.

In a worst case scenario, some estimates place the death toll from a direct hit in the tens of thousands.

If the surge were to breach the city’s levee system, the water could rise to as high as 20 feet in places, Mayor Ray Nagin told CNN Sunday.

That much water would overwhelm the city’s pump system and could take weeks to remove, Nagin said.

“We are facing the storm that we have feared “ Nagin said.

Thousands of residents of South Louisiana jammed highways Sunday morning in an exodus to higher and safer ground, but Nagin said at least 100,000 New Orleans residents rely solely on public transportation and have no means of leaving the city.

They’re being urged to head to pick up points for transportation to the New Orleans Superdome, which Nagin says is the shelter of last resort.

Firefighters and police officers were driving through New Orleans neighborhoods Sunday morning, trying to make sure that no one was left behind in the effort to empty the city before Katrina makes landfall Monday.

If Katrina doesn’t lose strength, it would be only the fourth Category Five storm to hit the U.S. since record keeping began.

The last was Hurricane Andrew in 1992, which killed 43 people in southern Florida and caused $31 billion in damage.

The Louisiana National Guard is on alert, but thousands of guard troops from the state are now serving in Iraq.

Nagin said 1,500 troops are immediately available, however, and another2,500 have been mobilized.

Residents of Mississippi and Alabama are also bracing for the storm.

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